Since Reed Wickner first proposed that otherwise obscure non-Mendelian states in S. cerevisiae, [URE3] and [PST], result from prion-like conversions of endogenous proteins, fungal prions in general and [PST] in particular, have proven to be uniquely powerful systems for exploring universal features of prion biology. A few ofthe major insights include direct demonstrafion ofthe protein only hypothesis of prion inheritance, elucidafion of molecular basis of prion strains as being enciphered in the conformation ofthe infectious protein, discovery of a proliferation of novel prion proteins and the suggestion that prion-based inheritance could be a ubiquitous mechanism for epigenetic control of protein function, and revelation that the host chaperone machinery plays a crifical role in catalyzing prion replication. As with all experiments, these findings also raise a host of fundamental questions such as the structural basis ofthe different strain conformations, how these different conformations are inherited and why do they differentially impact a cell's physiology, and perhaps most profoundly what is the biological role of prions. We now have the tools and the intellectual foundafion to begin to provide clear and concrete answers to these questions. These insights in turn should directly inform efforts to understand the principles of prion infectivity for the mammalian PrP protein and more generally to advance our understanding of how and why proteins misfold and how such misfolded forms impact a cell in both disease and non disease states. To accomplish this, we propose to focus on the following three areas: Define the structural basis of prion strain variants. Define how the primary structure of prion determines the spectrum of preferred strain variants Use ribosome profiling to define the physiological impact ofthe [PST] prion and how the prion strain conformation as well as the genetic background ofthe yeast modulate these effects. RELEVANCE (See instructions): Protein misfolding is a hallmark of a wide range of neurodegenerative disorders including prion diseases and far more common noninfectious disease such as Alzheimers and Parkinson.